Plasmas consist of gaseous complexes in which atoms or molecules are dissociated into free electrons, ions, free radicals, and neutral particles. Stars, for instance, consist predominantly of plasmas. On earth, plasmas occur naturally in lightning bolts, flames, and similar high-energy phenomena, or may be manufactured by heating a gas to high temperatures, or by applying a strong electric field to a gas. Plasmas are called the “fourth state of matter” because their physical properties make them physically distinct from solids, liquids, and gases.
Ions, as well as electrons, from various kinds of plasma generators can be used in such industrial processes as etching, ashing (as with photoresist material or surfaces being chemically machined), deposition of materials such as oxides or nitrides, oxidation, sputtering, polymerization, ion implantation within surfaces and also in high-specific-impulse thrusters for use on satellites and other space vehicles.
Drawbacks of existing direct current (DC) ion sources include erosion, short service life of plasma generators, and plasma non-uniformity. Erosion derives from the impacting of high-speed ions on the surfaces of the machines that produce plasmas. For example, DC ion sources eject erosion products into the discharge plasma as a consequence of the fact that the discharge cathode is constantly being bombarded by the ions of the plasma in which it is immersed. This is an undesirable attribute from the standpoint of materials processing, as contamination of the work product can result. DC ion sources (and DC electron sources) have limited lifetimes due to being constantly subjected to erosion, and the cathodes that drive such plasma sources typically, over time, lose their ability to emit electrons so that eventually the cathodes fail. Typically, DC ion sources (ion thrusters in particular) utilize a single on-axis discharge cathode, which gives rise to peaked, non-uniform plasma density profiles at the exit plane. Such non-uniform profiles cause non-uniform wear of the ion extraction grids—thereby leading to failure by structural degradation or by electron backstreaming.
Disk shaped multi-slotted antenna designs have been used in the past to circumvent the aforesaid issues. These sources require, however, an insulating window for operation, i.e., for impedance matching and shielding. The insulating window, typically boron nitride makes such devices impractical for ion sources or ion thruster applications because the insulating window acquires over time a coating due to wear of the extraction grids. Said coating will ultimately prevent microwaves form penetrating the source and thus plasma production will cease.
The production of large-area plasmas that are also large in volume and provide dense plasmas is much sought after in the area of electric propulsion and plasma processing. Achieving these plasma characteristics is generally difficult from the standpoint of issues such as recombination, collisional losses and diffusion, all of which reduce discharge efficiency and uniformity of the discharged reaction mass. Moreover, the design of plasma generators that are intended for use in electric propulsion and plasma processing applications tends toward the production of plasma discharges having minimal internal erosion of the source. From an electric propulsion thruster standpoint, this design goal provides extended operation lifetime. For plasma processing, it reduces the amount of contamination of the materials being processed.
It is also important that plasma discharges take place at reduced pressures. Hollow cathode based sources in a multipole configuration can be implemented to generate reasonably large discharge plasmas. However, such discharges tend to be of poor uniformity and to introduce erosion products due to cathode degradation (as it is exposed to the discharge plasma and bombarded by high-energy discharge ions). In this respect, conventional hollow cathode based discharge sources are not a solution to long life and low erosion plasma sources.
The prior art evidenced in patent literature shows various microwave, permanent magnet, ECR plasma sources, but they suffer from limitations that the present invention overcomes.
U.S. Patent Application 2004/0045674 A1 to Ishii, et al., “Radial Antenna and Plasma Device Using It,” describes a general microwave discharge, not an electron cyclotron resonance discharge (ECR). In this system, the microwave discharge is fundamentally limited in maximum plasma density, efficiency, and pressure. It is not an efficient ion source at the kinds of low pressures that are desirable for directional etching and sputter deposition applications in microelectronics. The invention of Ishii, et al., also uses a dielectric window, which can be problematic for both ion thruster uses and many deposition microelectronics reactors where metal vapor is present in the plasma. Metal ions and atoms can condense on the window, forming a layer that eventually prevents any microwave power from entering the system. Additionally, the device of Ishii, et al., utilizes a coaxial line connection to the slotted antenna, which limits the amount of power, plasma density and thus the maximum dimension to which the source can be built, thus limiting the ability to scale it up without recourse to a complete system redesign to scale up to a larger size.
U.S. Patent Application 2003/0183170 A1, to Kato, et al., “Plasma Processing Apparatus,” also describes a microwave system that lacks the potential of ECR. The comments above, in relation to US 2004/0045674 A1 apply to this source as well.
U.S. Patent Application 2003/0173030, to Ishii, et al., “Plasma Processing Apparatus,” describes essentially the same device addressed in relation to US 2004/0045674 A1. In this case however, Ishii, et al., focus on plasma processing application of the device. In this regard, its size is limited and can be scaled up only with difficulty.
U.S. Patent Application 2002/0121344 A1, to Noguchi, “Plasma Generating Device and Plasma Processing Apparatus Comprising Such a Device,” utilizes the same physics described in the patents described above. Power is fed to it by means of a coaxial line.
Japanese Patent 06151092 A, to Kyoichi, “Microwave Plasma Treatment Device,” also describes a microwave discharge device that is similar to the ones taught in the foregoing patents. It does not describe a high density low pressure ECR source.
Japanese Patent 06158298 A, to Mutsumi, et al., “Plasma Treating Device,” does not describe microwave plasma of any sort. It describes a RF glow discharge for plasma processing applications. Such devices operated at pressures ˜1 Torr and plasma densities are low and not particularly suited for etching or Sputter deposition. Sputter contamination is an issue for such a source.
WO 91/12353, “Device for Treating Substrates in a Gas-Based Plasma Produced by Microwaves,” describes a specialized microwave plasma source intended for the processing of optical coatings. It suffers from limitations described above in comments 1-4.
U.S. Pat. No. 5,324,362, to Schneider, et al., “Apparatus for Treating Substrates in a Microwave-Generated Gas-Supported Plasma,” apparently refers to a US patent WO 91/12353. This technology suffers from limitations described above in comments 1-4. As a sputtering source it could introduce contaminants in a deposition or etching plasma. It also presents a lifetime issue as the antenna would be subject to sputtering. The source also utilizes a microwave window, which has disadvantages described herein.
U.S. Pat. No. 6,376,028, to Laurent, et al. “Device and Method for Treating the Inside Surface of a Plastic Container with a Narrow Opening in a Plasma Enhanced Process,” does not describe a plasma source but rather a device and process that requires a plasma (preferably microwave generated). It is not applicable to the present invention.
U.S. Pat. No. 6,153,977, to Taira, et al., “ECR Type Plasma Generating Apparatus,” refers to an ECR source that utilizes a helical antenna that presumably launches a directed microwave beam toward and ECR zone established by two permanent magnets in opposition. It is inherently a small diameter device, and the ECR zone must be established between two closely spaced magnets. The device is not scalable to larger dimensions of the sort useful for large area plasma processing, high current, or long life ion thruster applications. Moreover, it is limited with respect to plasma density, which means that a workpiece to be processed must rely on the diffusion of the magnetized plasma, which is in general a slow process and can result in non-uniformities. And because it has an internal antenna it will be subject to sputter erosion limitations on service life, while also generating contaminants. The outer ceramic shield would be subject to the formation of metal coatings over time, which could affect the microwave coupling and thus the overall operation. Also because the device is coaxially fed, it is inherently limited to reduced microwave power.
U.S. Pat. No. 5,707,452, to Dandl, “Coaxial Microwave Applicator for an Electron Cyclotron Resonance Plasma Source,” describes a permanent magnet ECR source that utilizes internal coaxially fed antennas immersed in ECR zones to produce plasma. This use of the coax fed antennas circumvents issues of a similar device patented by Dandl: U.S. Pat. No. 5,203,960 and U.S. Pat. No. 5,370,765 which utilize internal antennas that are subject to erosion and therefore become likely plasma contamination sources. Additionally, as each internal antenna is coaxially fed, which makes them power limited.
U.S. Pat. No. 5,203,960, to Dandl, “Method of Operation of Electron Cyclotron Resonance Plasma Source,” and U.S. Pat. No. 5,370,765, also to Dandl, “Electron Cyclotron Resonance Plasma Source and Method of Operation,” cannot be utilized efficiently at lower, more commercially assessable frequencies such as 2.45 GHZ. Patent '960 has cylindrical geometry which means that scaling to larger volumes requires a complete redesign of the magnetic circuit.
U.S. Pat. No. 6,322,662, to Ishii, et al. “Plasma Treatment System,” utilizes a coax fed slotted antenna which inherently limits power and complicates implementation, as the coax feed would necessarily be water cooled at modest powers. It also uses a ceramic microwave window which would be subject to coating and so preclude its application to etching and deposition plasmas where metal vapors could be deposited on the ceramic. Additionally, the slotted antenna geometry of this invention is complicated and its overall layout does not lend well to scaling up in power. The antenna geometry is sophisticated, thereby imposing or requiring significant fabrication effort. Additionally, this invention is not an ECR source, but rather utilizes microwave energy to directly sustain the discharge via pair production. In this regard, it has to operate at a high background pressures that limit its uses. In general, the devices described in the Dandl patents, by virtue of the plasma production approach, will likely not scale with increasing diameter. The ECR zones are not couple via the ring cusp magnetic circuit, which allows for very large area/volume plasma production with straightforward scaling.